This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and or 365 to 193873/1997 filed in Japan on Jul. 18, 1997; the entire content of which is hereby incorporated by reference.
This invention relates to a mold made of a ceramic matrix and used for molding glass optical elements such as lenses. More particularly, this invention relates to a mold usable upon reproduction and a method for reproducing the mold.
A mold used for press molding glass molded articles, which are not subject to grinding or polishing during a cold state, transfers the molding surface of the mold onto the glass surface as it is during press molding. Therefore, it is required for such a molding surface of a mold, e.g., to be capable of being finished to an optical mirror surface, to be free from rough surfaces due to oxidization even at a high temperature, and to have a mechanical strength durable against impacts when press molding.
Such a mold has been developed in various ways. For examples, Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-45,135 has proposed xe2x80x9ca mold having upper and lower molds for press molding a glass molding material, the mold characterized in that a xcex2 silicon carbide mainly having 111 face directionality is covered on a mold substrate surface for the mold corresponding to surfaces of the glass molding material.xe2x80x9d
Moreover, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-83,529 discloses xe2x80x9ca method for manufacturing a glass mold characterized in after a substrate material for a glass mold is fabricated into a shape corresponding to a shape of a glass molded article to be produced, a hard carbon film is formed on the substrate material by a sputter method using a graphite as a sputter target in an inert gas as a sputter gas at a substrate material temperature of 250 to 450xc2x0 C.xe2x80x9d
This Publication includes the following description. A silicon carbide sintered material is used as a substrate material, and after this is roughly fabricated to a shape corresponding to a shape of the glass molded article to be produced, a silicon carbide film (thickness of 500 microns) is formed on the substrate material surface by a CVD (Chemical Vapor Deposition) method. After the surface of the silicon carbide film is finished to a shape of the glass molded article to be produced, a hard carbon film is formed thereon by a sputter method. It is described that where the hard carbon film is not formed, the glass recognizably adheres by press molding of several times, but where the hard carbon film (1000 Angstroms thickness) is formed, the glass does not adhere by press molding of 150 times, and first adherence is recognized by press molding of 200 to 300 times. Because the hard carbon film can be removed easily by oxygen plasma ashing, it is described that the mold can be advantageously reused upon reproduction by repeating removals and depositions of films.
Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-38,330 discloses an excellent adherence force is obtainable by a treatment of the molding surface of the mold with a hydrogen fluoride or an aqueous solution of its salt after the hard carbon film is removed by oxygen plasma ashing before another new hard carbon film is formed.
The silicon carbide film made by the CVD method has features that the film is dense, can be finished to an optical mirror surface, and is free from rough surfaces because, though a very surface layer is oxidized, oxidation hardly develops at high temperature. However, if the glass material is pressed so that the silicon carbide film is directly in contact with the glass material, the glass may adhere to the silicon carbide film, and the silicon carbide film may be locally scraped out (hereinafter, referred to as xe2x80x9cpulloutsxe2x80x9d).
Although mold releasing capability can be improved by formation of a mold releasing film such as a hard carbon film, such a hard carbon film is not a permanent film and requires repeating of removal and formation every about 200 times. For example, if a formed film is inhomogeneous or does not have a long life due to deviations of film formation conditions, press molding may cause occurrences of pullouts. When pullouts occur, such a mold cannot be used as it is because lens would be produced with failures to its appearance. During removal of the film, the carbon film is removed by ashing with oxygen plasma, and at that time, the surface of the silicon carbide is oxidized. Because the carbon film would lose its adherence force at the subsequent film formation if the surface of the silicon carbide were oxidized, the subsequent film is formed upon removing the oxidized layer in solving the layer with a hydrogen fluoride or an aqueous solution of its salt. Rough surfaces would occur on the surface of the silicon carbide by repeating these steps. If glass optical elements are pressed using a mold having rough surfaces, the glass molded article (lenses) may have blurs.
Thus, the molding surface made of the silicon carbide by the CVD method has relatively short life. It is therefore desired to refinish only the molding surface of the mold which already became useless and to reuse the mold. The silicon carbide film made by the CVD method normally has, even if thickly formed, a thickness of several hundreds microns, and the remaining thickness after processed is around 200 to 300 microns, and if rough surfaces are created, the film can be reproduced once or twice. However, since pullouts are formed deeply, most of the silicon carbide film may be removed by grinding if the molding surface is ground to delete the pullouts.
It is also conceivable that another silicon carbide film would be newly stacked by the CVD method on the mold to be reproduced. In this situation, however, the CVD method makes depositions not only on the molding surface of the mold made of a silicon carbide sintered material as a substrate but also on side and bottom surfaces. If the silicon carbide is formed again on the entire surface of the mold to be reproduced, the side and bottom surfaces are required to be re-fabricated with high accuracy, thereby raising costs higher. Even if the side and bottom surfaces are masked as not to form the silicon carbide, the gas may enter in the spaces, and it is difficult to protect the surfaces completely. Therefore, a process is required in which the silicon carbide film deposited on the side and bottom surfaces is removed upon fabrication, and the mold cannot be reproduced merely by forming a silicon carbide film only on a molding surface.
It is therefore an object of the invention to provide a mold easily reusable even where a molding surface is subject to impairments such as pullouts by repetitive use.
It is another object of the invention to provide a method for manufacturing glass optical elements using a mold whose molding surface can be reproduced.
This invention relates to a mold (hereinafter referred to as xe2x80x9cfirst moldxe2x80x9d) having upper and lower molds for obtaining glass optical elements by press molding a glass molding material softened by heat, the mold characterized in that at least one of the upper and lower molds is made of a ceramic matrix, which has no surface hole having a diameter of 300 microns or more on a molding surface.
This invention also relates to a mold (hereinafter referred to as xe2x80x9csecond moldxe2x80x9d) having upper and lower molds for obtaining glass optical elements by press molding a glass molding material softened by heat, the mold characterized in that a portion constituting a molding surface of at least one of the upper and lower molds has a thickness such that the molding surface can be reproduced by grinding and is made of a xcex2 type silicon carbide having a density of 3.20 g/cm3 or more.
This invention further relates to a method for manufacturing glass optical elements in repeating a process for press molding a glass molding material softened by heat with a mold to obtain glass optical elements, the method characterized in that the mold is made of a ceramic matrix and has a molding surface reproduced by at least grinding and that the reproduced molding surface has no surface hole having a diameter of 300 microns or more.
Moreover, this invention relates to a method for manufacturing glass optical elements in repeating a process for press molding a glass molding material softened by heat with a mold to obtain glass optical elements, the method characterized in that a portion constituting a molding surface of at least one of the upper and lower molds constituting the mold has a thickness such that the molding surface can be reproduced by grinding and that the mold has a molding surface reproduced at least by grinding and is made of a ceramic matrix made of a xcex2 type silicon carbide having a density of 3.20 g/cm3 or more.
This invention also relates to a method for reproducing a mold, including upper and lower molds for obtaining glass optical elements by press molding a glass molding material softened by heat, having a portion constituting a molding surface of at least one of the upper and lower molds of a thickness such that the molding surface can be reproduced by grinding, and being made of a xcex2 type silicon carbide having a density of 3.20 g/cm3 or more, the method characterized in that the impaired molding surface due to repeating of press molding is reproduced by grinding the molding surface.